How the Moon Was Born

The Coaccretion Theory of the Moon's Origin

Another theory for the origin of the moon discussed at the 1964 conference was that the moon formed together with the Earth at the same time and in the same place. Just as the Earth was believed to have been created by accretion (the accumulation of solid particles and larger objects) about 4.6 billion years ago when the solar system was taking shape, so, according to this theory, was the moon.

But the “coaccretion” theory presented problems as well. Astronomers had calculated the moon's density by using a formula based on the moon's mass, as determined by its distance from the Earth and its orbital velocity, divided by its volume. The moon's density, calculated to be 3.3 times the density of liquid water, is much less than the density of the Earth, which is 5.5 times the density of water. If the Earth and the moon were formed from the same cloud of matter, at the same distance from the sun, how could their composition be so different?

In addition, other doubts plagued scientists as well. If the moon formed near Earth by the same processes that formed the other planets, why don't the other planets of the inner solar system have large satellites like the moon? Mercury and Venus have no moons, and Mars has only two tiny satellites that may be captured asteroids.

For such reasons, some scientists by the time of the 1964 conference were already doubtful about the validity of the coaccretion theory as well as the earlier theory that the moon formed at about the same distance from the sun as the Earth. Instead, most participants at the meeting favored some version of Urey's theory in which the moon was formed elsewhere and then captured by the Earth, or some version of the fission theory. After all, they noted, because fission would take place in the upper layers of the Earth, which are much less dense than its heavy iron core, fission could account for the moon's low density and apparent lack of a significant iron core. Though fission caused by Earth's rapid rotation had been ruled out, the scientists speculated that a portion of the Earth might still have been flung off to form the moon in some other way.